---
title: "AgentSubscriber"
description:
  "Event-driven subscriber system for agent lifecycle and event handling"
---

# AgentSubscriber

The `AgentSubscriber` interface provides a comprehensive event-driven system for
handling agent lifecycle events, message updates, and state mutations during
agent execution. It allows you to hook into various stages of the agent's
operation and modify its behavior.

```typescript
import { AgentSubscriber } from "@ag-ui/client"
```

## Overview

`AgentSubscriber` defines a collection of optional event handlers and lifecycle
hooks that can respond to different stages of agent execution. All methods in
the interface are optional, allowing you to implement only the events you need
to handle.

All subscriber methods can be either synchronous or asynchronous - if they
return a Promise, the agent will await their completion before proceeding.

## Adding Subscribers to Agents

Subscribers can be added to agents in two ways:

### Permanent Subscription

Use the `subscribe()` method to add a subscriber that will persist across
multiple agent runs:

```typescript
const agent = new HttpAgent({ url: "https://api.example.com/agent" })

const subscriber: AgentSubscriber = {
  onTextMessageContentEvent: ({ textMessageBuffer }) => {
    console.log("Streaming text:", textMessageBuffer)
  },
}

// Add permanent subscriber
const subscription = agent.subscribe(subscriber)

// Later, remove the subscriber if needed
subscription.unsubscribe()
```

### Temporary Subscription

Pass a subscriber directly to `runAgent()` for one-time use:

```typescript
const temporarySubscriber: AgentSubscriber = {
  onRunFinishedEvent: ({ result }) => {
    console.log("Run completed with result:", result)
  },
}

// Use subscriber for this run only
await agent.runAgent({ tools: [myTool] }, temporarySubscriber)
```

## Core Interfaces

### AgentSubscriber

The main interface that defines all available event handlers and lifecycle
hooks. All methods in the interface are optional, allowing you to implement only
the events you need to handle.

### AgentStateMutation

Event handlers can return an `AgentStateMutation` object to modify the agent's
state and control event processing:

```typescript
interface AgentStateMutation {
  messages?: Message[] // Update the message history
  state?: State // Update the agent state
  stopPropagation?: boolean // Prevent further subscribers from processing this event
}
```

When a subscriber returns a mutation:

- **messages**: Replaces the current message history
- **state**: Replaces the current agent state
- **stopPropagation**: If `true`, prevents subsequent subscribers from handling
  the event (useful for overriding default behavior)

### AgentSubscriberParams

Common parameters passed to most subscriber methods:

```typescript
interface AgentSubscriberParams {
  messages: Message[] // Current message history
  state: State // Current agent state
  agent: AbstractAgent // The agent instance
  input: RunAgentInput // The original input parameters
}
```

## Event Handlers

### Lifecycle Events

#### onRunInitialized()

Called when the agent run is first initialized, before any processing begins.

```typescript
onRunInitialized?(params: AgentSubscriberParams): MaybePromise<Omit<AgentStateMutation, "stopPropagation"> | void>
```

#### onRunFailed()

Called when the agent run encounters an error.

```typescript
onRunFailed?(params: { error: Error } & AgentSubscriberParams): MaybePromise<Omit<AgentStateMutation, "stopPropagation"> | void>
```

#### onRunFinalized()

Called when the agent run completes, regardless of success or failure.

```typescript
onRunFinalized?(params: AgentSubscriberParams): MaybePromise<Omit<AgentStateMutation, "stopPropagation"> | void>
```

### Event Handlers

#### onEvent()

General event handler that receives all events during agent execution.

```typescript
onEvent?(params: { event: BaseEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onRunStartedEvent()

Triggered when an agent run begins execution.

```typescript
onRunStartedEvent?(params: { event: RunStartedEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onRunFinishedEvent()

Called when an agent run completes successfully.

```typescript
onRunFinishedEvent?(params: { event: RunFinishedEvent; result?: any } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onRunErrorEvent()

Triggered when an agent run encounters an error.

```typescript
onRunErrorEvent?(params: { event: RunErrorEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onStepStartedEvent()

Called when a step within an agent run begins.

```typescript
onStepStartedEvent?(params: { event: StepStartedEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onStepFinishedEvent()

Triggered when a step within an agent run completes.

```typescript
onStepFinishedEvent?(params: { event: StepFinishedEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

### Message Events

#### onTextMessageStartEvent()

Triggered when a text message starts being generated.

```typescript
onTextMessageStartEvent?(params: { event: TextMessageStartEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onTextMessageContentEvent()

Called for each chunk of text content as it's generated.

```typescript
onTextMessageContentEvent?(params: { event: TextMessageContentEvent; textMessageBuffer: string } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onTextMessageEndEvent()

Called when a text message generation is complete.

```typescript
onTextMessageEndEvent?(params: { event: TextMessageEndEvent; textMessageBuffer: string } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

### Tool Call Events

#### onToolCallStartEvent()

Triggered when a tool call begins.

```typescript
onToolCallStartEvent?(params: { event: ToolCallStartEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onToolCallArgsEvent()

Called as tool call arguments are being parsed, providing both raw and parsed
argument data.

```typescript
onToolCallArgsEvent?(params: { event: ToolCallArgsEvent; toolCallBuffer: string; toolCallName: string; partialToolCallArgs: Record<string, any> } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onToolCallEndEvent()

Called when a tool call is complete with final arguments.

```typescript
onToolCallEndEvent?(params: { event: ToolCallEndEvent; toolCallName: string; toolCallArgs: Record<string, any> } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onToolCallResultEvent()

Triggered when a tool call result is received.

```typescript
onToolCallResultEvent?(params: { event: ToolCallResultEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

### State Events

#### onStateSnapshotEvent()

Called when a complete state snapshot is provided.

```typescript
onStateSnapshotEvent?(params: { event: StateSnapshotEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onStateDeltaEvent()

Triggered when partial state changes are applied.

```typescript
onStateDeltaEvent?(params: { event: StateDeltaEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onMessagesSnapshotEvent()

Called when a complete message history snapshot is provided.

```typescript
onMessagesSnapshotEvent?(params: { event: MessagesSnapshotEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onActivitySnapshotEvent()

Called when an activity snapshot is received. The handler receives both the raw
event and any existing `ActivityMessage` (if present) so you can inspect or
replace it before the default client logic runs.

```typescript
onActivitySnapshotEvent?(params: {
  event: ActivitySnapshotEvent
  activityMessage?: ActivityMessage
  existingMessage?: Message
} & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onActivityDeltaEvent()

Triggered for each activity delta. Use this hook to transform or debounce the
incoming JSON Patch operations before they update the conversation transcript.

```typescript
onActivityDeltaEvent?(params: {
  event: ActivityDeltaEvent
  activityMessage?: ActivityMessage
} & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onRawEvent()

Handler for raw, unprocessed events.

```typescript
onRawEvent?(params: { event: RawEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

#### onCustomEvent()

Handler for custom application-specific events.

```typescript
onCustomEvent?(params: { event: CustomEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>
```

### State Change Handlers

#### onMessagesChanged()

Called when the agent's message history is updated.

```typescript
onMessagesChanged?(params: Omit<AgentSubscriberParams, "input"> & { input?: RunAgentInput }): MaybePromise<void>
```

#### onStateChanged()

Triggered when the agent's state is modified.

```typescript
onStateChanged?(params: Omit<AgentSubscriberParams, "input"> & { input?: RunAgentInput }): MaybePromise<void>
```

#### onNewMessage()

Called when a new message is added to the conversation.

```typescript
onNewMessage?(params: { message: Message } & Omit<AgentSubscriberParams, "input"> & { input?: RunAgentInput }): MaybePromise<void>
```

#### onNewToolCall()

Triggered when a new tool call is added to a message.

```typescript
onNewToolCall?(params: { toolCall: ToolCall } & Omit<AgentSubscriberParams, "input"> & { input?: RunAgentInput }): MaybePromise<void>
```

## Async Support

All subscriber methods support both synchronous and asynchronous execution:

```typescript
const subscriber: AgentSubscriber = {
  // Synchronous handler
  onTextMessageContentEvent: ({ textMessageBuffer }) => {
    updateUI(textMessageBuffer)
  },

  // Asynchronous handler
  onStateChanged: async ({ state }) => {
    await saveStateToDatabase(state)
  },

  // Async handler with mutation
  onRunInitialized: async ({ messages, state }) => {
    const enrichedState = await loadUserPreferences()
    return {
      state: { ...state, ...enrichedState },
    }
  },
}
```

## Multiple Subscribers

Agents can have multiple subscribers, which are processed in the order they were
added:

```typescript
// First subscriber modifies state
const stateEnricher: AgentSubscriber = {
  onRunInitialized: ({ state }) => ({
    state: { ...state, timestamp: new Date().toISOString() },
  }),
}

// Second subscriber sees the modified state
const logger: AgentSubscriber = {
  onRunInitialized: ({ state }) => {
    console.log("State after enrichment:", state)
  },
}

agent.subscribe(stateEnricher)
agent.subscribe(logger)
```

## Integration with Agents

Basic usage pattern:

```typescript
const agent = new HttpAgent({ url: "https://api.example.com/agent" })

// Add persistent subscriber
agent.subscribe({
  onTextMessageContentEvent: ({ textMessageBuffer }) => {
    updateStreamingUI(textMessageBuffer)
  },
  onRunFinishedEvent: ({ result }) => {
    displayFinalResult(result)
  },
})

// Run agent (subscriber will be called automatically)
const result = await agent.runAgent({
  tools: [myTool],
})
```
